Oxide semiconductor substrate and schottky barrier diode

ABSTRACT

A Schottky barrier diode element includes an n-type or p-type silicon (Si) substrate, an oxide semiconductor layer, and a Schottky electrode layer, the oxide semiconductor layer including either or both of a polycrystalline oxide that includes gallium (Ga) as the main component and an amorphous oxide that includes gallium (Ga) as the main component.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/912,800, filed Feb. 18, 2016, which is a U.S. National StageEntry of International Patent Application No. PCT/JP2014/004154, filedAug. 8, 2014, which claims the benefit of priority from Japanese PatentApplication No. 2013-169967, filed Aug. 19, 2013, and Japanese PatentApplication No. 2014-030250, filed Feb. 20, 2014, the entireties ofwhich are all hereby incorporated herein by reference.

TECHNICAL FIELD

The invention relates to an oxide semiconductor substrate and a Schottkybarrier diode element that have rectifying characteristics.

BACKGROUND ART

A Schottky barrier diode is a diode that produces a rectifying actionutilizing a potential barrier formed at the junction between a metal anda semiconductor. Si is most widely used as the semiconductor (see PatentLiterature 1, for example). As a compound semiconductor having a bandgap wider than that of Si, GaAs and, recently, SiC may be used insteadof Si (see Patent Literatures 2 and 3, for example).

An Si-based Schottky diode is used for a high-speed switching element, aGHz-band transmission/reception mixer, a frequency conversion element,and the like. A GaAs-based Schottky diode can implement a higher-speedswitching element, and is used for a microwave converter, a microwavemixer, and the like. It is considered that SiC can be applied to theelectric vehicle field, the railroad field, the power transmissionfield, and the like (for which a higher voltage is required) due to itswide band gap.

A Schottky barrier diode that utilizes Si is relatively inexpensive, andis widely used. However, since the band gap of Si is as narrow as 1.1eV, it is necessary to increase the size of the element in order toimprove the breakdown characteristics. The band gap of GaAs is 1.4 eV,which is wider than that of Si. However, since it is difficult toimplement the epitaxial growth of GaAs on an Si substrate, it isdifficult to obtain a crystal with a small number of dislocations. Sincethe band gap of SiC is as wide as 3.3 eV, it is considered that a highdielectric breakdown field and better performance can be achieved byutilizing SiC. However, since the substrate production process and theepitaxial growth process require a high-temperature process, SiC has aproblem from the viewpoint of mass productivity and cost.

Recently, Ga₂O₃ has attracted attention as a material having a band gapwider than that of SiC.

An oxide semiconductor is a material that has high mobility and a wideenergy gap, and the application of an oxide semiconductor to anext-generation display driver transistor, a short-wavelength sensor, alow-power-consumption circuit, and the like has been desired. Non-PatentLiterature 1 reports that monoclinic β-Ga₂O₃ was used for a powerdevice, and VB=0.71 MV/cm was achieved. Patent Literature 4 discloses anexample in which an ohmic electrode obtained by stacking monoclinicβ-Ga₂O₃ and Ti is applied to a light-emitting diode.

Ga₂O₃ may have an α, β, γ, δ, or ∈ crystal structure. Ga₂O₃ having a βcrystal structure (monoclinic crystal structure) has the highest thermalstability, and it has been reported that the band gap of Ga₂O₃ having aβ crystal structure is 4.8 eV to 4.9 eV. A β-Ga₂O₃ monocrystallinesubstrate can be obtained using a floating zone (FZ) method or anedge-defined film-fed growth (EFG) method. However, since it isnecessary to use a molecular beam epitaxy method at present in order toimplement homoepitaxial growth, there is a problem from the viewpoint ofmass productivity.

CITATION LIST Patent Literature

-   Patent Literature 1: JP-A-2009-164237-   Patent Literature 2: JP-A-H5-36975-   Patent Literature 3: JP-A-H8-97441-   Patent Literature 4: Japanese Patent No. 5078039

Non-Patent Literature

-   Non-Patent Literature 1: K. Sasaki et al., Appl. Phys. Express    5 (2012) 035502

SUMMARY OF INVENTION

The invention was conceived in view of the above problems. An object ofthe invention is to provide a Schottky barrier diode element that isobtained by forming a compound semiconductor having a wide band gap onan inexpensive substrate (e.g., Si wafer) at low cost with high massproductivity, and that exhibits excellent current-voltagecharacteristics.

Another object of the invention is to provide an oxide semiconductorsubstrate that is suitable for a Schottky barrier diode element, a diodeelement, and a power semiconductor element.

The invention provides the following Schottky barrier diode element andthe like.

1. A Schottky barrier diode element including an n-type or p-typesilicon (Si) substrate, an oxide semiconductor layer, and a Schottkyelectrode layer, the oxide semiconductor layer including either or bothof a polycrystalline oxide that includes gallium (Ga) as the maincomponent and an amorphous oxide that includes gallium (Ga) as the maincomponent.2. A Schottky barrier diode element including an n-type or p-typesilicon (Si) substrate, an oxide semiconductor layer, and a Schottkyelectrode layer, the oxide semiconductor layer including apolycrystalline oxide that includes gallium (Ga) as the main component.3. The Schottky barrier diode element according to 1 or 2, wherein theoxide semiconductor layer includes gallium at an atomic percentage([Ga]/([Ga]+[total metal elements other than Ga])×100) of 90 to 100 at%.4. The Schottky barrier diode element according to any one of 1 to 3,wherein the oxide semiconductor layer is formed on the siliconsubstrate, and the Schottky electrode layer is formed on the oxidesemiconductor layer.5. The Schottky barrier diode element according to any one of 1 to 3,wherein the Schottky electrode layer is formed on the silicon substrate,and the oxide semiconductor layer is formed on the Schottky electrodelayer.6. The Schottky barrier diode element according to any one of 1 to 5,wherein the oxide semiconductor layer includes at least one elementselected from Si, Ge, Sn, Ti, Zr, and Hf in a ratio of 0.01 at % to 10at % based on the total metal elements included in the oxidesemiconductor layer.7. The Schottky barrier diode element according to any one of 1 to 6,wherein the oxide semiconductor layer has a carrier concentration of1×10¹⁴ cm⁻³ or more and 1×10¹⁷ cm⁻³ or less at room temperature.8. The Schottky barrier diode element according to any one of 1 to 7,wherein the Schottky electrode layer is a metal thin film having a workfunction of 4.7 eV or more.9. The Schottky barrier diode element according to any one of 1 to 8,wherein the oxide semiconductor layer is covered with an insulating filmso that the edge of the oxide semiconductor layer is not exposed.10. An electric circuit including the Schottky barrier diode elementaccording to any one of 1 to 9.11. An electric apparatus including the Schottky barrier diode elementaccording to any one of 1 to 9.12. An electronic apparatus including the Schottky barrier diode elementaccording to any one of 1 to 9.13. A vehicle including the Schottky barrier diode element according toany one of 1 to 9.14. A structure including a metal thin film having a work function of4.7 eV or more, and an oxide semiconductor that includes Ga as the maincomponent, the structure having a region in which the metal thin filmand the oxide semiconductor electrically contact with each other.15. The structure according to 14, wherein the oxide semiconductor thatincludes Ga as the main component includes at least one element selectedfrom Si, Ge, Sn, and Ti in a ratio of 0.01 at % or more and 10 at % orless based on the total metal elements included in the oxidesemiconductor.16. The structure according to 14 or 15, wherein the oxide semiconductorincludes gallium at an atomic percentage ([Ga]/([Ga]+[total metalelements other than Ga])×100) of 90 to 100 at %.17. The structure according to any one of 14 to 16, wherein the oxidesemiconductor has a carrier concentration of 1×10¹⁴ cm⁻³ or more and1×10¹⁷ cm⁻³ or less at room temperature.18. The structure according to any one of 14 to 17, wherein the oxidesemiconductor has a thickness of 50 nm to 20 μm.19. The structure according to any one of 14 to 18, wherein the metalthin film is formed of Au, Cr, Cu, Fe, Ir, Mo, Nb, Ni, Pd, Pt, Re, Ru,W, In₂O₃, In—Sn—O, or In—Zn—O.20. An oxide semiconductor substrate including a conductive substrate,and the structure according to any one of 14 to 19 that is stacked onthe conductive substrate.21. The oxide semiconductor substrate according to 20, wherein theconductive substrate is formed of one or more materials selected frommonocrystalline silicon, polycrystalline silicon, and microcrystallinesilicon.22. An oxide semiconductor substrate including an insulating substrate,and the structure according to any one of 14 to 19 that is stacked onthe insulating substrate.23. A power semiconductor element wherein the oxide semiconductorsubstrate according to any one of 20 to 22 is used.24. A diode element wherein the oxide semiconductor substrate accordingto any one of 20 to 22 is used.25. A Schottky barrier diode element wherein the oxide semiconductorsubstrate according to any one of 20 to 22 is used.26. A Schottky barrier diode element including the oxide semiconductorsubstrate according to any one of 20 to 22, the oxide semiconductor thatincludes Ga as the main component being an oxide semiconductor layer,and the metal thin film having a work function of 4.7 eV or more being aSchottky electrode layer.27. An electric circuit including one or more elements selected from thegroup consisting of the power semiconductor element according to 23, thediode element according to 24, and the Schottky barrier diode elementaccording to 25 or 26.28. An electric apparatus including the electric circuit according to27.29. An electronic apparatus including the electric circuit according to27.30. A vehicle including the electric circuit according to 27.

The invention thus provides a Schottky barrier diode element that isobtained by forming a compound semiconductor having a wide band gap onan Si substrate at low cost with high mass productivity, and thatexhibits excellent current-voltage characteristics.

The invention also thus provides an oxide semiconductor substrate thatis suitable for a Schottky barrier diode element, a diode element, and apower semiconductor element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating oneembodiment of the Schottky barrier diode element according to theinvention.

FIG. 2 is a cross-sectional view schematically illustrating oneembodiment of the Schottky barrier diode element according to theinvention.

FIG. 3 is a cross-sectional view schematically illustrating oneembodiment of the Schottky barrier diode element according to theinvention.

FIG. 4 is a cross-sectional view schematically illustrating an oxidesemiconductor substrate including the structure according to theinvention obtained in Example 7.

FIG. 5 is a graph illustrating the current-voltage characteristics ofthe structure obtained in Example 7.

FIG. 6 is a cross-sectional view schematically illustrating the Schottkybarrier diode element according to the invention obtained in Example 8.

FIG. 7 is a graph illustrating the current-voltage characteristics ofthe Schottky barrier diode element according to the invention obtainedin Example 8.

FIG. 8 illustrates the X-ray diffraction chart (XRD) of the oxidesemiconductor film obtained in Example 8.

FIG. 9 illustrates the X-ray diffraction chart (XRD) of the oxidesemiconductor film obtained in Example 10.

FIG. 10 illustrates the X-ray diffraction chart (XRD) of the oxidesemiconductor film obtained in Comparative Example 2.

DESCRIPTION OF EMBODIMENTS 1. Schottky Barrier Diode Element

The Schottky barrier diode element according to the invention includesan n-type or p-type silicon (Si) substrate, an oxide semiconductorlayer, and a Schottky electrode layer, the oxide semiconductor layerincluding either or both of a polycrystalline oxide that includesgallium (Ga) as the main component and an amorphous oxide that includesgallium (Ga) as the main component.

The expression “the oxide semiconductor layer includes a polycrystallineoxide that includes gallium (Ga) as the main component” used hereinmeans that the oxide semiconductor layer includes gallium at an atomicpercentage ([Ga]/([Ga]+[total metal elements other than Ga])×100) of 90to 100 at %. The term “polycrystalline oxide” used herein refers to anaggregate of Ga₂O₃ crystals in which the crystallographic directions arenot necessarily identical. The term “amorphous oxide” used herein refersto an oxide for which a diffraction peak is not observed by X-raydiffraction analysis.

It is possible to provide a Schottky barrier diode element that hasexcellent current-voltage characteristics (particularly a highdielectric breakdown field) and high mass productivity by utilizing agallium oxide-based polycrystalline material having a wide band gap.

The oxide semiconductor layer preferably includes gallium at an atomicpercentage of 90 at % or more, and more preferably 95 at % or more,based on the total metal elements included in the oxide semiconductorlayer. In this case, it is considered that the wide band gap of Ga₂O₃ ismaintained, and a high breakdown voltage is achieved. The upper limit ofthe gallium content is not particularly limited. For example, the upperlimit of the gallium content is 100 at %.

The oxide semiconductor layer may further include one or more elementsselected from Si, Ge, Sn, Ti, Zr, and Hf. Specifically, the oxidesemiconductor layer includes gallium oxide (Ga₂O₃), and optionallyincludes an oxide of these additional elements. The oxide of theseadditional elements is not particularly limited.

The additional element is preferably one or more elements selected fromSi, Sn, Ti, and Zr.

The compositional ratio of the elements included in the oxidesemiconductor layer may be quantitatively determined by secondary ionmass spectrometry (SIMS). More specifically, after the cross section ofthe semiconductor layer is exposed by polishing or the like, and thecompositional ratio of the elements included in the oxide semiconductorlayer is quantitatively determined by a calibration curve method using astandard sample having a known concentration.

When the oxide semiconductor layer is formed using a sputtering method,the compositional ratio of the elements included in the oxidesemiconductor layer is almost identical to the composition of thesputtering target. The compositional ratio of the elements included inthe sputtering target is determined by quantitatively analyzing theelements using an inductively coupled plasma atomic emissionspectrometer (ICP-AES).

Specifically, when a solution sample is nebulized using a nebulizer, andintroduced into an argon plasma (about 6,000 to 8,000° C.), each elementincluded in the sample absorbs the thermal energy and is excited, andthe orbital electrons migrate from the ground state to the orbital at ahigh energy level. The orbital electrons then migrate to the orbital ata lower energy level when about 10⁻⁷ to 10⁻⁸ seconds has elapsed. Inthis case, the difference in energy is emitted as light. Since theemitted light has an element-specific wavelength (spectral line), thepresence or absence of each element can be determined based on thepresence or absence of the spectral line.

More specifically, a solution sample prepared by dissolving thesputtering target in a solvent using an acid treatment is subjected toquantitative determination by a calibration curve method using astandard sample having a known concentration, and the concentration ofeach element included in the solution is converted into the content(composition) (at %) of each element included in the target.

Since the intensity (luminous intensity) of each spectral line is inproportion to the number of respective elements included in the sample,the concentration of each element in the sample can be determinedthrough a comparison with a standard solution having a knownconcentration.

After specifying the elements included in the sample by qualitativeanalysis, the content of each element is determined by quantitativeanalysis, and the atomic ratio of each element is calculated from theresults.

The gallium oxide used for the Schottky barrier diode element accordingto the invention is polycrystalline and/or amorphous. When the galliumoxide is polycrystalline, the gallium oxide may have an α, β, γ, δ, or ∈crystal form, or may be a mixture thereof. Note that it is preferablethat the gallium oxide include β-Ga₂O₃ as the main component from theviewpoint of operational stability.

Pure polycrystalline Ga₂O₃ has a wide band gap, but has a low carrierconcentration at room temperature, and an increase in On-resistanceoccurs when it is operated as a diode. Heat may be generated when theOn-resistance is high. Such a problem can be reduced by dopingpolycrystalline Ga₂O₃ with an appropriate amount of a positivetetravalent element such as one or more elements selected from Si, Ge,Sn, Ti, Zr, and Hf.

The amount of doping with these additional elements is preferably 0.01at % to 10 at %, and more preferably 0.04 to 5 at %, based on the totalmetal elements included in the oxide semiconductor layer. If the amountof doping is less than 0.01 at %, the carrier concentration remains lowin spite of doping. If the amount of doping exceeds 10 at %, segregationmay occur at the grain boundaries of polycrystalline Ga₂O₃, and thedielectric breakdown field intensity may decrease when a reverse bias isapplied.

The additional element(s) may be doped using a method that mixes Ga₂O₃into the sputtering target, a method that effects cosputtering using adoping oxide target and Ga₂O₃, a method that effects ion doping with thedesired donor atom after forming a Ga₂O₃ film, or the like. Note thatthe method that mixes Ga₂O₃ into the sputtering target has an advantagein that a uniform doping concentration profile can be obtained (i.e.,high productivity can be achieved). The ion doping method has anadvantage in that the doping profile can be controlled to a certainextent by controlling the accelerating voltage and time. For example, itis possible to improve the performance of the diode by doping an areasituated at the interface with the Schottky electrode layer with theadditional element(s) at a low concentration, and doping an areasituated at the interface with the ohmic electrode layer with theadditional element(s) at a high concentration.

The carrier concentration in the gallium oxide used for the Schottkybarrier diode element according to the invention may also be adjusted byincorporating an oxide of one or more elements selected from Zn, In, Cd,Al, and Mg in the oxide semiconductor layer in a ratio of 0.01 at % to10 at % based on the total metal elements included in the oxidesemiconductor layer. This method has an effect of adjusting the band gapinstead of doping Ga₂O₃ with an additional element. When ZnO, In₂O₃,CdO, or SnO₂ is added, the band gap of Ga₂O₃ becomes narrower, and thecarrier concentration increases. When Al₂O₃ or MgO is added, the bandgap of Ga₂O₃ becomes wider, and the carrier concentration decreases.

The band gap and the carrier concentration are parameters that determinethe breakdown voltage and the On-resistance of the Schottky barrierdiode. An optimum band gap and an optimum carrier concentration differdepending on the application. When a low On-resistance is importantrather than the breakdown characteristics (breakdown voltage), the bandgap may be narrowed as compared with that of Ga₂O₃. When the breakdowncharacteristics (breakdown voltage) are important rather than a lowOn-resistance, the band gap may be widened as compared with that ofGa₂O₃. The On-resistance and the breakdown voltage normally have atrade-off relationship. However, it is possible to adjust theOn-resistance and the breakdown characteristics (breakdown voltage) in awell-balanced manner by utilizing an oxide semiconductor as comparedwith the case of using a known silicon-based material.

It is preferable that the oxide semiconductor layer have a carrierconcentration of 1×10¹⁴ cm⁻³ or more and 1×10¹⁷ cm⁻³ or less at roomtemperature (298K) in order to obtain good diode characteristics. If thecarrier concentration is less than 1×10¹⁴ cm⁻³, the On-resistance mayincrease to a large extent, and generation of heat may occur duringoperation. If the carrier concentration exceeds 1×10¹⁷ cm⁻³, theOn-resistance may decrease to a large extent, and a leakage current whena reverse bias is applied may increase. The carrier concentration ismore preferably 1×10¹⁵ cm⁻³ or more and 5×10¹⁶ cm⁻³ or less. The carrierconcentration is measured using the method described in connection withthe examples.

An n-type silicon substrate and a p-type silicon substrate may be usedas the silicon (Si) substrate. A known silicon substrate that exhibitsexcellent surface flatness-smoothness (e.g., monocrystalline siliconsubstrate, polycrystalline silicon substrate, and microcrystallinesilicon substrate) may be used as the silicon substrate.

Note that a microcrystal is classified as a polycrystal. A polycrystalis an aggregate of single crystals, and has clear grain boundaries,which may affect electrical characteristics. A microcrystal has asubmicrometer particle size, and has no clear grain boundaries.Therefore, a microcrystal has an advantage in that a variation inelectrical characteristics due to grain boundary scattering occurs toonly a small extent.

The Schottky electrode layer is formed using a material having a workfunction of 4.7 eV or more. Specific examples of such a material includeRu, Au, Pd, Ni, Ir, Pt, and alloys thereof. If the work function is lessthan 4.7 eV, the height of the Schottky barrier may be low, and leakagemay significantly occur when a reverse bias is applied.

On the other hand, the work function of a metal used for an ohmicelectrode layer is preferably about 4.1 eV depending on the impurityconcentration in a silicon wafer, and Ti and Mo are preferable takingaccount of adhesion.

In one embodiment of the Schottky barrier diode element according to theinvention, the oxide semiconductor layer is formed on the siliconsubstrate, and the Schottky electrode layer is formed on the oxidesemiconductor layer.

When using an n-type silicon wafer, a Ga₂O₃-based oxide semiconductor isstacked on the front side of the substrate, and an electrode layer(e.g., Pt, Au, Pd, or Ni) that forms a Schottky barrier is stacked onthe Ga₂O₃-based oxide semiconductor. An electrode layer (e.g., Ti) thatforms an ohmic junction with n-type silicon is stacked on the back sideof the substrate. It is preferable to stack a good conductor (e.g., Au)on the back side of the substrate through Ni in order to ensureconduction. Note that Ni prevents the diffusion of Au.

In another embodiment of the Schottky barrier diode element according tothe invention, the Schottky electrode layer is formed on the siliconsubstrate, and the oxide semiconductor layer is formed on the Schottkyelectrode layer.

When using a p-type silicon wafer, the Schottky electrode layer (e.g.,Pt, Au, Pd, or Ni) is stacked on the front side of the substrate, and aGa₂O₃-based oxide semiconductor is formed on the Schottky electrodelayer using a sputtering method. In this case, a Schottky barrier isformed at the interface between the metal (e.g., Pt, Au, Pd, or Ni) andthe oxide semiconductor layer. It is possible to obtain better diodecharacteristics by oxidizing the surface of the Schottky electrode layerusing an oxygen plasma, UV ozone, or the like before forming the oxidesemiconductor layer.

When the oxide semiconductor layer is formed by a sputtering methodusing pure Ga₂O₃, it is preferable to dope the oxide semiconductor layerby ion doping with a positive tetravalent element such as one or moreelements selected from Si, Ge, Sn, Ti, Zr, and Hf. The oxidesemiconductor layer is mainly doped on the surface thereof, and it isnecessary to adjust the field intensity during doping so that theSchottky interface is not reached. After completion of ion doping, theoxide semiconductor layer is annealed at 200° C. or higher and 600° C.or lower in order to effect activation.

A metal (e.g., Ti) that forms an ohmic junction with the oxidesemiconductor layer is stacked on the oxide semiconductor layer. In thiscase, a good conductor (e.g., Au) may be further stacked through Ni. Anelectrode that exhibits excellent adhesion and assists in conduction isstacked on the back side of the p-type silicon wafer.

A known guard ring structure may be provided to the Schottky barrierdiode element according to the invention. The guard ring is providedbetween the oxide semiconductor layer and the Schottky electrode layer,and has an effect of increasing the breakdown voltage. An electric fieldmay be concentrated on the edge of the oxide semiconductor layer so thata dielectric breakdown may easily occur. It is possible to furtherincrease the breakdown voltage by stacking an insulating film (e.g.,SiO₂) so as to cover the edge of the oxide semiconductor layer.

It is preferable that the Schottky barrier diode element according tothe invention have a structure in which the oxide semiconductor layer iscovered with an insulating film so that the edge thereof is not exposed.

The oxide semiconductor layer, the Schottky electrode layer, the ohmicelectrode layer, and the like that form the Schottky barrier diodeelement according to the invention may be formed using a knownsputtering method or the like that can be implemented at low cost withhigh mass productivity (see the examples).

A thin oxide film having a thickness of 10 nm or less may be stacked atthe interface between the electrode layer that forms the Schottkyelectrode and the oxide semiconductor layer by performing reactivesputtering when forming the Schottky electrode by sputtering whileintroducing oxygen.

After forming the oxide semiconductor layer, the oxide semiconductorlayer may be annealed to effect crystallization. It is possible to lowerthe On-resistance and prevent generation of heat by crystallizing theoxide semiconductor. The annealing conditions are not particularlylimited. For example, after forming the oxide semiconductor layer, theoxide semiconductor layer is allowed to stand at 500° C. for 0.5 hoursin nitrogen to stabilize the oxidation state. After forming theelectrode layer, the oxide semiconductor layer is annealed at 200° C.for 1 hour in air. Whether or not the oxide semiconductor has beencrystallized may be determined by X-ray diffraction (XRD) measurement orusing a TEM.

Note that it is preferable to use the oxide semiconductor layer in anamorphous state when a crystal grain boundary, a lattice defect, and thelike may be produced due to polycrystallization, and may decrease thebreakdown voltage. When using the oxide semiconductor layer in anamorphous state, the oxide semiconductor layer may be heated at 300° C.or less for 1 hour or less taking account of the types of the elementsthat form the oxide semiconductor layer. It is possible to obtain astable amorphous state by heating the oxide semiconductor layer at atemperature as low as 300° C. or less.

The Schottky barrier diode element according to the invention has a highdielectric breakdown field. The dielectric breakdown field of theSchottky barrier diode element according to the invention is preferably0.5 MV/cm or more, and more preferably 0.7 MV/cm or more. In this case,since it is possible to design the diode to have a small thickness, itis possible to reduce the size of the element, and it is advantageous interms of heat dissipation.

The n-value of the Schottky barrier diode element according to theinvention is preferably 2 or less, and more preferably 1.5 or less. Inthis case, it is possible to lower the On-resistance, and reduce orsuppress generation of heat.

The Schottky barrier diode element according to the invention issuitably used for an electric circuit, an electric apparatus, anelectronic apparatus, a vehicle, and an electric vehicle.

2. Structure and Oxide Semiconductor Substrate

The structure according to the invention includes a metal thin filmhaving a work function of 4.7 eV or more, and an oxide semiconductorthat includes Ga as a main component, the structure having a region inwhich the metal thin film and the oxide semiconductor electricallycontact with each other.

When the structure according to the invention is applied to a Schottkybarrier diode element, the metal thin film having a work function of 4.7eV or more functions as a Schottky electrode layer, and the oxidesemiconductor that includes Ga as the main component functions as anoxide semiconductor layer.

The oxide semiconductor substrate according to the invention includes aconductive substrate, and the structure according to the invention thatis stacked on the conductive substrate.

The oxide semiconductor substrate according to the invention is anintermediate that is useful for producing a Schottky barrier diodeelement, a power semiconductor element, and a diode element.

The Schottky barrier diode element according to the invention thatachieves the above object includes a structure that has a region inwhich a metal thin film having a work function of 4.7 eV or more and anoxide semiconductor that includes Ga as the main component electricallycontact with each other.

The expression “a metal thin film and an oxide semiconductorelectrically contact with each other” used herein refers to a contactstate in which the metal thin film and the oxide semiconductor film forma junction so that electrons can freely diffuse from the oxidesemiconductor into the metal thin film such that the metal thin film andthe oxide semiconductor film are identical as to Fermi energy. Theregion in which the metal thin film and the oxide semiconductorelectrically contact with each other may be a region in which the metalthin film and the oxide semiconductor are bonded directly in a state inwhich an insulating film or the like is not interposed therebetween.

It is preferable that the oxide semiconductor that includes Ga as themain component include at least one element selected from Si, Ge, Sn,Ti, Hf, and Zr in a ratio of 0.01 at % or more and 10 at % or less basedon the total metal elements included in the oxide semiconductor.

It is preferable that the oxide semiconductor that includes Ga as themain component include gallium at an atomic percentage([Ga]/([Ga]+[total metal elements other than Ga])×100) of 90 to 100 at%.

It is preferable that the oxide semiconductor that is included in thestructure and includes Ga as the main component have a carrierconcentration of 1×10¹⁴ cm⁻³ or more and 1×10¹⁷ cm⁻³ or less at roomtemperature (298K). The carrier concentration is more preferably 1×10¹⁵cm⁻³ or more and 5×10¹⁶ cm⁻³ or less.

The carrier concentration may be determined using the measurement methoddescribed in connection with the examples.

Examples of the metal thin film having a work function of 4.7 eV or moreinclude a film formed of a metal such as Au, Cr, Cu, Fe, Ir, Mo, Nb, Ni,Pd, Pt, Re, Ru, or W, a film formed of a metal oxide such as In₂O₃, ITO(In—Sn—O), or IZO (In—Zn—O), and the like. Note that it is advantageousto use a metal having a larger work function and a higher carrierconcentration in order to obtain clear rectifying characteristics. Thework function is more preferably 4.8 eV or more, and still morepreferably 5.0 eV or more. The upper limit of the work function is notparticularly limited. For example, the upper limit of the work functionis 5.6 eV.

When using a metal oxide as the material for forming the metal thinfilm, it is preferable that the metal have a carrier concentration of10²⁰ cm⁻³ or higher. If the carrier concentration is lower than 10²⁰cm⁻³, the extent of a depletion layer may increase when the metal thinfilm is stacked on the oxide semiconductor that includes Ga as the maincomponent, and internal resistance may occur, or high-speed switchingcharacteristics may be impaired. It is more preferable to use Au, Ir,Ni, Pd, or Was the material for forming the metal thin film when themetal thin film is stacked on the oxide semiconductor that includes Gaas the main component.

A trace amount of metal may be added to these materials to such anextent that the work function does not decrease in order to improveworkability. For example, an alloy to which Ag and Cu are added may beused when Au is used as the material for forming the metal thin film,and an alloy to which Ag and Cu are added may be used when Pd is used asthe material for forming the metal thin film.

The work function is measured using a photoelectron spectrometer (e.g.,“AC-3” manufactured by Riken Keiki Co., Ltd.). The work function changesdue to a surface treatment (e.g., acid or alkali surface treatment), UVcleaning, and the like. Note that the term “work function” used hereinrefers to a value measured directly after a film has been formed withoutperforming an additional treatment.

At least one element selected from Si, Ge, Sn, and Ti may be added tothe oxide semiconductor that includes Ga as the main component. Theconcentration (total concentration) of the additional element(s) in theoxide semiconductor is preferably 0.01 at % or more and 10 at % or lessbased on the total metal elements included in the oxide semiconductor.If the total concentration of the additional element(s) added to theoxide semiconductor is 0.01 at % or less, the carrier concentration inGa₂O₃ may decrease, and an increase in electrical resistance may occur.If the total concentration of the additional element(s) added to theoxide semiconductor exceeds 10 at %, the conduction path of Ga₂O₃ thatprovides electrical conduction may be interrupted, and a decrease inmobility and an increase in electrical resistance may occur. Therefore,the forward voltage may increase, and power loss or generation of heatmay occur when the oxide semiconductor to which at least one elementselected from Si, Ge, Sn, and Ti is added at a concentration outside theabove range is used.

The oxide semiconductor that includes Ga as the main component that isused in connection with the invention may have an arbitrary structure(e.g., monocrystalline, amorphous, or polycrystalline structure). It isconsidered that the On-resistance becomes a minimum when the oxidesemiconductor has a monocrystalline structure. Note that the oxidesemiconductor need not necessarily have a monocrystalline structure.Since the oxide semiconductor that includes Ga as the main component hasa wide band gap as compared with crystalline Si as well as SiC and GaNthat are used for a next-generation power device, the oxidesemiconductor has a high dielectric breakdown field. Therefore, it ispossible to achieve a moderate On-resistance and a high breakdownvoltage by reducing the film thickness when a very high voltage is notrequired. Note that it is necessary to suppress a situation in whichcrystal growth occurs to an excessive extent when using an oxidesemiconductor having a polycrystalline structure since leakage may occurthrough the grain boundaries.

The oxide semiconductor that includes Ga as the main component that isused in connection with the invention may be formed by applying a vacuumvapor-phase method such as a sputtering method, a vacuum depositionmethod, or a CVD method, a normal pressure vapor-phase method such as anatmospheric pressure CVD method, a spray pyrolysis method, or a mist CVDmethod, a liquid-phase method such as a spin coating method, an inkjetmethod, a casting method, a micelle disruption method, or anelectrodeposition method, or the like. Note that it is also possible touse an epitaxial growth method (e.g., laser ablation, MBE, or MOCVD) asa film-forming method that forms a single crystal so as to have the samelattice constants as those of the substrate.

However, since the oxide semiconductor that includes Ga as the maincomponent that is used in connection with the invention may have anarbitrary structure, an epitaxial method need not necessarily be used inview of productivity and mass productivity. It is advantageous toincrease the film thickness in order to obtain a higher breakdownvoltage. A normal pressure gas-phase method and a liquid-phase methodare advantageous from the viewpoint of obtaining a relatively thickfilm. However, impurities may be mixed, and a moderate heat treatment isrequired when using a normal pressure gas-phase method or a liquid-phasemethod. Note that a micelle disruption method, an electrodepositionmethod, and the like achieves excellent throwing power, and it isconsidered that leakage can be prevented when these methods are appliedto a diode.

A CVD method such as a thermal CVD method, a CAT-CVD method, a photo-CVDmethod, a mist CVD method, an MO-CVD method, or a plasma CVD method, anatomic-level film-forming method such as an MBE method or an ALD method,a PVD method such as an ion plating method, an ion-beam sputteringmethod, or a magnetron sputtering method, a method that utilizes a knownceramic process, such as a doctor blade method, an injection method, anextrusion method, a hot press method, a sol-gel method, or an aerosoldeposition method, a wet method such as a coating method, a spin coatingmethod, a printing method, a spray method, an electrodeposition method,a plating method, or a micelle disruption method, or the like may beused as the thin film-forming method. The dielectric breakdown field ofthe oxide semiconductor used for the Schottky barrier diode (Schottkybarrier diode element) according to the invention is 0.5 to 3 MV/cm,which is significantly higher than that of a known silicon-based diode.The desired breakdown voltage differs depending on the application andthe objective. A film thickness of 0.2 μm to 1.2 μm is required when abreakdown voltage of 60 V is desired, and a film thickness of 2 μm to 12μm is required when a breakdown voltage of 600 V is desired. Inparticular, when it is desired to form a film having a thickness of 2 μmor more, it is advantageous to use a CVD method or a wet method ascompared with a PVD method from the viewpoint of the production process.

The thickness of the oxide semiconductor is preferably 50 nm or more and20 μm or less. If the thickness of the oxide semiconductor is less than50 nm, the breakdown voltage decreases to about 10 V, which isinsufficient for most applications. A breakdown voltage of 5,000 V maybe achieved when the thickness of the oxide semiconductor is larger than20 μm. In this case, however, generation of heat may occur duringswitching due to an increase in On-resistance. The thickness of theoxide semiconductor is more preferably 200 nm or more and 12 μm or less.

The thickness of the oxide semiconductor may be measured using a stylusprofilometer (e.g., Surfcorder or DEKTAK) or an electron microscope(e.g., SEM or TEM).

The structure according to the invention may be stacked on a conductivesubstrate, or may be stacked on an insulating substrate. Note that it ispossible to achieve better heat dissipation by stacking the structureaccording to the invention on a conductive substrate. A known substratethat exhibits excellent surface flatness-smoothness (e.g.,monocrystalline silicon substrate, polycrystalline silicon substrate, ormicrocrystalline silicon substrate) may be used as the conductivesubstrate.

The oxide semiconductor substrate according to the invention is requiredto exhibit surface flatness-smoothness. When the oxide semiconductorsubstrate is used in the vertical direction, electrical conductivity isalso required for the oxide semiconductor substrate. A silicon substratecan meet the above conditions at low cost. Note that a substrate formedof a metal such as Cu, Al, Mo, W, Ni, Cr, Fe, Nd, Au, Ag, Nd, or Pd, oran alloy thereof may also be used. In particular, it is considered thata heat dissipation effect is achieved when a metal material having highthermal conductivity is used. A heat sink structure may be employed whenfurther heat dissipation is required. A compound monocrystalline wafer(e.g., GaAs and InP), and a substrate formed an oxide, a nitride, acarbide, and the like (e.g., Al₂O₃, ZnO, MgO, SrTiO₃, YSZ, lanthanumaluminate, Y₃Al₅O₁₂, NdGaO₃, sapphire, AlN, GaN, SiC, alkali-free glass,and soda lime glass) may also be used. When the oxide semiconductorsubstrate is used in the lateral direction, an insulating substrate mayalso be used. Note that the term “vertical direction” used herein meansthat a current is caused to flow in the direction perpendicular to thesurface of the oxide semiconductor, and the term “lateral direction”used herein means that a current is caused to flow in the directionhorizontal to the surface of the oxide semiconductor.

Examples of the insulating substrate include a glass substrate and aresin substrate (e.g., polycarbonate, polyarylate, polyethyleneterephthalate, polyethersulfone, polyimide, and phenolic resin).

Since the structure according to the invention does not require ahigh-temperature process, a power supply for a circuit that drives adisplay (e.g., liquid crystal display or organic EL display) can beprovided on the same substrate as the display, for example.

When staking an ohmic electrode on the oxide semiconductor substrateaccording to the invention, it is preferable to select a material thathas a work function close to the work function (3.7 eV to 4.3 eV) of theoxide semiconductor that includes Ga₂O₃ as the main component. The workfunction of the oxide semiconductor that includes Ga₂O₃ as the maincomponent varies depending on the type and the concentration of eachelement. It is preferable to use Ti as the material for forming theohmic electrode taking account of adhesion.

It is preferable that the oxide semiconductor that includes Ga₂O₃ as themain component used in connection with the invention have an amorphousstructure or a polycrystalline structure. When using an oxidesemiconductor having a polycrystalline structure, leakage may easilyoccur through the grain boundaries due to excessive crystal growth.

After forming the Ga₂O₃ film, an annealing treatment may be performed tosuch an extent that the growth of a polycrystal does not occurexcessively. The annealing treatment reduces the abstraction of oxygenin the subsequent step that stacks the ohmic electrode. If the annealingtreatment is not performed, oxygen may migrate from Ga₂O₃ to the ohmicelectrode in the step that stacks the ohmic electrode, and the carrierconcentration in the Ga₂O₃ region may increase. If the abstraction ofoxygen reaches the Schottky region, the rectifying effect may be lost.

The oxide semiconductor substrate according to the invention is suitablyused for a power semiconductor element, a diode element, and a Schottkybarrier diode element. An electric circuit that includes one or moreelements among the power semiconductor element, the diode element, andthe Schottky barrier diode element is suitably used for an electricdevice, an electronic device, and an electric vehicle.

The invention provides a stack that is suitable as a member that forms apower semiconductor element (diode element, IGBT element, and MOSFET).In particular, the invention can advantageously provide a Schottkybarrier diode element, a PN diode element, and a PIN diode element.

It is possible to reduce or suppress generation of heat and reduce powerconsumption by applying the invention to a rectifier diode used for apower supply circuit, a fast recovery diode used for a PWM invertercircuit, and the like. In particular, an inverter circuit is required tohave a high operating frequency and a short switching recovery time.Since it is possible to implement a reduction in thickness and amonopolar structure as compared with a known fast recovery diode, it ispossible to significantly reduce the recovery time. Therefore, it ispossible to more effectively utilize the features of the diode accordingto the invention as the operating frequency increases.

For example, a GTO has been used as an inverter circuit used forvehicles. The GTO is suitable for a high-power switching operation.However, since the frequency is about 500 Hz, significant noise occursat the time of start. Therefore, an IGBT has been increasingly providedto a vehicle and an EV in recent years. The switching speed of the IGBTcan be increased to several tens of kHz, and it is possible to reduce orsuppress noise, and reduce the size of a peripheral member. Inprinciple, the switching loss of the IGBT is small. However, since theoperating frequency of the IGBT is high, it is significantly effectiveto reduce the reverse leakage current of the fast recovery diode that isused in combination with the IGBT in order to reduce power consumption.Therefore, the diode according to the invention for which the reverseleakage current is smaller than that of a known Si diode is particularlyeffective as a fast recovery diode used for an IGBT inverter. The effectfurther increases when it is desired to increase the operating frequencyand achieve smoother operation. Since it is also possible to reduce orsuppress generation of heat, it is possible to further simplify thecooling mechanism. For example, a plurality of cooling mechanismsrequired for an EV can be replaced by a 110° C. radiator.

EXAMPLES

The invention is further described below by way of examples withappropriate reference to the drawings.

Example 1

FIG. 1 is a cross-sectional view schematically illustrating a Schottkybarrier diode element obtained in Example 1.

An n-type silicon (Si) substrate 11 having a resistivity of 0.02 Ω·cmwas provided, and treated with diluted hydrofluoric acid to remove anatural oxide film from the surface of the substrate. The Si substrate(wafer) was placed in a sputtering device (“HSM552” manufactured byShimadzu Corporation). A sputtering discharge was effected at an RFpower of 100 W using a Ga₂O₃ sintered body including 500 ppm of Si (thiscomposition is hereinafter referred to as “Si—Ga₂O₃”) as the sputteringtarget to form an Si—Ga₂O₃ film (gallium oxide film) 12 having athickness of 300 nm on the surface of the Si substrate from which anoxide film was removed.

The Si—Ga₂O₃ film was patterned by photolithography to form the desiredpattern, and annealed at 500° C. for 0.5 hours in nitrogen to effectcrystallization. The crystal state of the Si—Ga₂O₃ film was determinedby XRD measurement. The Si substrate on which the polycrystallineSi—Ga₂O₃ film was formed was placed in the sputtering device, andsputtering was performed using a Pt target to form a Pt electrode 13 onthe polycrystalline Si—Ga₂O₃ film (i.e., a Schottky junction wasformed).

The substrate was immersed in diluted hydrofluoric acid to remove anatural oxide film from the surface of the substrate on which thepolycrystalline Si—Ga₂O₃ film was not formed, and a Ti film 14, an Nifilm 15, and an Au film 16 were sequentially formed by sputtering toform an ohmic electrode. The resulting stack was annealed at 200° C. for1 hour in air to obtain a Schottky barrier diode element 10.

CV (capacitance-voltage) measurement was performed in order to determinethe carrier concentration in the Si—Ga₂O₃ film at room temperature. Thedepletion layer capacitance C (F/cm²) per unit area is represented byC=∈/W. Note that ∈ is the dielectric constant (F/cm) of thesemiconductor, and W is the width (cm) of the depletion layer. When aforward bias voltage V (V) is applied to the Schottky diode, the widthof the depletion layer is represented by W={2∈(φ−V)/qN}(½). Therefore,the depletion layer capacitance per unit area is represented byC={q∈N/2(φ−V)}(½). Note that q is the elementary charge(=1.6×10⁻¹⁹ (C)),and φ is the built-in potential (V) that represents the difference incontact potential between the Pt electrode and the Si—Ga₂O₃ film.

The C⁻²-V characteristics determined by the CV measurement are plotted,and the doping concentration (=carrier concentration) N is calculatedfrom the slope of the C⁻²-V characteristics. The carrier concentrationcalculated from the slope of the C⁻²-V characteristics was 5×10¹⁵ cm⁻³.

The current-voltage characteristics of the resulting Schottky barrierdiode element were measured, and the n-value and the reverse breakdownvoltage were calculated. Note that the n-value is a parameter thatrepresents the characteristic of a Schottky barrier diode element (seethe following expression (1)). Ideal element characteristics areobtained when the n-value is close to 1.

I=I0[exp(eV/nkT)]  (1)

I: Total current density (A/cm²) of current that flows from galliumoxide film toward Si substrate

e: Carrier (1.60×10⁻¹⁹ (C))

V: Voltage (V) applied to elementI0: Current density (A/cm²) when voltage (V) applied to element is 0 Vk: Boltzmann constant (1.38×10⁻²³ (J/K))

T: Temperature (K)

The n-value was calculated to be 1.7, and the reverse breakdown voltagewas calculated to be 23 V. This reverse breakdown voltage corresponds toan dielectric breakdown field of 0.77 MV/cm, which is higher than thatof a known Schottky barrier diode that utilizes monocrystalline Si by afactor of about 2.

Note that the reverse breakdown voltage is calculated by “reversebreakdown voltage (V)=dielectric breakdown field (V/cm)×thickness (cm)of semiconductor”.

The results are shown in Table 1. Note that “Forward voltage” in Table 1is a voltage required to achieve a current density of 0.1 mA/cm², and“On-current density” in Table 1 is a current density when a voltage of10 V is applied.

Examples 2 and 3

A Schottky barrier diode element was produced using a sputtering methodin the same manner as in Example 1, except that the Schottky electrodeand the semiconductor composition were appropriately changed as shown inTable 1. The measurement results for the current-voltage characteristicsare shown in Table 1.

Example 4

An n-type Si substrate having a resistivity of 0.02 Ω·cm was provided,and treated with diluted hydrofluoric acid to remove a natural oxidefilm from the surface of the substrate. The Si substrate (wafer) wasplaced in a sputtering device (“HSM552” manufactured by ShimadzuCorporation). Ga₂O₃ was used as the sputtering target. A sputteringdischarge was effected at an RF power of 100 W to form a gallium oxidefilm having a thickness of 300 nm on the surface of the Si substratefrom which an oxide film was removed.

The Si substrate (wafer) on which the gallium oxide film was formed wasplaced in an ion doping implantation device, and doped with Si at aconcentration of 0.5 at %. The Si substrate was then annealed at 500° C.for 1 hour in air to activate Si and form a polycrystalline Ga₂O₃ film.The polycrystalline Ga₂O₃ film was patterned by photolithography to formthe desired pattern. The substrate was placed in the sputtering device,and sputtering was performed using a Pt target to form a Pt electrode onthe polycrystalline Ga₂O₃ film (i.e., a Schottky junction was formed).

The substrate was immersed in diluted hydrofluoric acid to remove anatural oxide film from the surface of the substrate on which thepolycrystalline Ga₂O₃ film was not formed, and a Ti film, an Ni film,and an Au film were sequentially formed by sputtering to form an ohmicelectrode. The resulting stack was annealed at 200° C. for 1 hour in airto obtain a Schottky barrier diode element.

The CV measurement was performed in the same manner as in Example 1. Then-value was calculated to be 1.3, and the reverse breakdown voltage wascalculated to be 30 V. This reverse breakdown voltage corresponds to andielectric breakdown field of 1.0 MV/cm, which is higher than that of aknown Schottky barrier diode that utilizes monocrystalline Si by afactor of about 3.

Example 5

FIG. 2 is a cross-sectional view schematically illustrating a Schottkybarrier diode element obtained in Example 5.

A p-type silicon substrate 21 having a resistivity of 0.02 Ω·cm wasprovided, and treated with diluted hydrofluoric acid to remove aspontaneous oxide film. Sputtering was then performed using an Ni targetto form an Ni electrode 22. The surface of the Ni electrode was oxidizedusing UV ozone, and sputtering was performed using a Ga₂O₃ targetincluding 1 wt % of Sn to form an Sn—Ga₂O₃ film 23 having a thickness of300 nm. The resulting stack was annealed at 500° C. for 0.5 hours innitrogen, and a Ti film 24, an Ni film 25, and an Au film 26 weresequentially formed on the Sn—Ga₂O₃ film by sputtering to form an ohmicelectrode.

After removing a natural oxide film from the surface of the p-typesilicon substrate (opposite to the surface on which the Ni electrode wasformed) by using a diluted hydrofluoric acid, a TiAl film 27 was formedby sputtering using a TiAl alloy as the target. The resulting stack wasannealed at 200° C. for 1 hour in air to obtain a Schottky barrier diodeelement 20. The resulting diode was opposite in polarity to the diodesof Examples 1 to 5. A forward bias is applied when the p-type siliconwafer is connected to the positive terminal of the power supply, and areverse bias is applied when the p-type silicon wafer is connected tothe negative terminal of the power supply.

The measurement results for the current-voltage characteristics areshown in Table 1.

Example 6

FIG. 3 is a cross-sectional view schematically illustrating a Schottkybarrier diode element obtained in Example 6.

An n-type Si substrate 31 having a resistivity of 0.02 Ω·cm wasprovided, and treated with diluted hydrofluoric acid to remove a naturaloxide film from the surface of the Si substrate. The Si substrate(wafer) was placed in a sputtering device (“HSM552” manufactured byShimadzu Corporation). Ga₂O₃ including 1 wt % of Zr (this composition ishereinafter referred to as “Zr—Ga₂O₃”) was used as the sputteringtarget. A sputtering discharge was effected at an RF power of 100 W toform a Zr—Ga₂O₃ film 32 having a thickness of 300 nm.

A negative resist (manufactured by AZ Electronic Materials) was appliedto the Zr—Ga₂O₃ film 32 using a spin coating method. The resist waspre-baked, exposed, developed, and post-baked to form a ring-like recesspattern at the edge of the Zr—Ga₂O₃ film. The Si substrate (wafer) wasplaced in the sputtering device, and an SiO₂ film having a thickness of50 nm was formed by sputtering (RF power: 100 W, 50 min) using SiO₂ asthe target. The Si substrate was immersed in a resist stripper to removeunnecessary resist together with the Zr—Ga₂O₃ film. A guard ring 37 forthe Zr—Ga₂O₃ film was thus formed. A Pt electrode 33 and an ohmicelectrode (Ti film 34, Ni film 35, and Au film 36) were formed in thesame manner as in Example 1 to obtain a Schottky barrier diode element30 provided with the guard ring.

The measurement results for the current-voltage characteristics areshown in Table 1. The Schottky barrier diode element 30 exhibitedbreakdown voltage characteristics better than those of the Schottkybarrier diode element 10 of Example 1 due to the effect of the guardring.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Schottky electrode Pt Pd Pd Pd Ni Pt Work function    5.6    5.4    5.4   5.4    5.1    5.6 Si wafer n-type n-type n-type n-type p-type n-typeOhmic electrode Ti/Ni/Au Ti/Ni/Au Ti/Ni/Au Ti/Ni/Au Ti/Ni/Au Ti/Ni/AuSemiconductor Ga₂O₃ + SiO₂ Ga₂O₃:TiO₂ = Ga₂O₃:SnO₂ = Ga₂O₃ + Si 0.5Ga₂O₃:SnO₂ = Ga₂O₃:ZrO₂ = composition (wt %) (500 ppm) 98:2 wt % 99:1 wt% wt % (ion 99:1 wt % 99:1 wt % dropping) Semiconductor Ga:Si = Ga:Ti =Ga:Sn = Ga:Si = Ga:Sn = Ga:Zr = composition (at %) 99.96:0.0499.77:0.023 98.8:1.2 99.38:0.62 99.8:1.2 99.24:0.76 Annealing after filmNitrogen, Nitrogen, Nitrogen, Air, 500° C. , Nitrogen, Nitrogen,formation of 500° C. , 0.5 h 400° C. , 0.5 h 500° C. , 0.5 h 1 h 500° C., 0.5 h 500° C. , 0.5 h semiconductor Annealing after SBD Air, 200° C. ,Air, 200° C. , Air, 200° C. , Air, 200° C. , Air, 200° C. , Air, 200° C., was formed 1 h 1 h 1 h 1 h 1 h 1 h Carrier concentration 5 × 10¹⁵ 2 ×10¹⁶ 2 × 10¹⁶ 2 × 10¹⁶ 1 × 10¹⁵ 1 × 10¹⁵ (cm⁻³) n-value    1.7    1.3   1.3    1.3    1.3    1.3 Dielectric breakdown    0.77    0.5    0.5   1.0    0.7    0.67 field (MV/cm) Semiconductor XRD PolycrystallinePolycrystalline Polycrystalline Polycrystalline PolycrystallinePolycrystalline measurement result Substrate Si wafer Si wafer Si waferSi wafer Si wafer Si wafer Contact electrode Si Si Si Si Si Si Workfunction (eV)    4.1    4.1    4.1    4.1    4.1    4.1 of ohmicelectrode Upper electrode Pt Pd Pd Pd Ni Pt Forward voltage (V)    0.8   0.7    0.7    0.7    1.2    1.2 On-current (A/cm²)  >10  >10  >10 >10  >10  >10 Thickness (nm)   300   300   300   300   300   300

Example 7

An n-type Si substrate (diameter: 4 inches) having a resistivity of 0.02Ω·cm was provided. The Si substrate (wafer) was placed in a sputteringdevice (“HSM552” manufactured by Shimadzu Corporation), and a Ti film(15 nm) and a Pd film (50 nm) were sequentially formed by sputteringusing a circular area mask. After exchanging the area mask, a Ga₂O₃:SnO₂film having a thickness of 200 nm was formed by sputtering (RF power:100 W, Ar 100%) using a sintered Ga₂O₃:SnO₂ (=99.9:0.1 wt %) target. Theresulting structure was placed on a hot plate, and annealed at 300° C.for 1 hour in air.

The current-voltage characteristics of the resulting structure wereevaluated as described below. A product “SCS-4200” manufactured by ToyoCorporation was used as a source meter. The source terminal wasconnected to the oxide semiconductor, and the drain terminal wasconnected to the Pd electrode. A tungsten needle was used as theterminal. A current that flowed through the element was measured whilechanging the drain voltage. The resulting current-voltagecharacteristics showed clear rectifying characteristics (see FIG. 5).Note that the work function of the Schottky electrode was measured usinga photoelectron spectrometer “AC-3” manufactured by Riken Keiki Co.,Ltd.

The Ga₂O₃:SnO₂ thin film was subjected to XRD measurement. As a result,no diffraction peak was observed except for the sample stage and the Sisubstrate (wafer), and it was found that the Ga₂O₃:SnO₂ thin film was anamorphous film.

The following XRD measurement conditions were used.

Device: “SmartLab” manufactured by Rigaku CorporationX-ray: Cu-Kα line (wavelength: 1.5406 Å, monochromatized using agraphite monochromator)2θ-θ reflection method, continuous scan (1.0°/min)Sampling interval: 0.02°

Slit DS, SS: ⅔°, RS: 0.6 mm Example 8

The structure produced in Example 7 was placed in the sputtering device,and a Ti film (50 nm) and an Au film (50 nm) were sequentially formed bysputtering using an area mask having a diameter of 1 mm. FIG. 6 is aview schematically illustrating the resulting stack. The current-voltagecharacteristics were evaluated in the same manner as in Example 7. Thecurrent density was calculated by dividing the amount of current by thearea of the hole of the area mask having a diameter of 1 mm, and foundto be 30 A/cm² or more (forward current). The forward turn-on voltage(Vf) significantly decreased (i.e., 2.5 V) due to the provision of theohmic electrode. The breakdown voltage when a reverse bias was appliedwas −30 V, and the dielectric breakdown field intensity was 1.5 MV/cm.

The results are summarized in Table 2.

Examples 9 to 16

A structure similar to that of Example 8 was evaluated while changingthe semiconductor material and the electrode material. The results areshown in Table 2. Note that the term “microcrystal” in Table 2 isclassified as a polycrystal.

In Example 10, an inexpensive polycrystalline Si wafer was used as thesubstrate. Pt was used for the Schottky electrode, and the composition“Ga₂O₃:SiO=99.9:0.1 wt %” was used for the semiconductor. After formingthe semiconductor film, the resulting stack was annealed at 400° C. for1 hour in air. The film had a microcrystalline structure.

In Example 11, an alkali-free glass substrate was used as the substrate.In Example 12, a polyimide substrate was used as the substrate. InExamples 13 and 14, a polycarbonate resin substrate coated with SiO₂(hard coat) was used as the substrate. These substrates are insulatingsubstrates. Since the element according to the invention can be producedwithout using a monocrystalline semiconductor, it is possible to producethe element using such a wide variety of substrates. In Example 14, thedielectric breakdown field decreased to 0.2 MV/cm since theconcentration of Ga based on the total metal elements forming thesemiconductor was 88.8 at % that is lower than the preferable range.However, a performance comparable to that of a diode level that utilizescrystalline silicon was obtained.

In Example 15, since the stack was annealed at 600° C. for 1 hour in airin the final step, a polycrystalline structure was formed. Therefore,the dielectric breakdown field decreased. However, the forward voltagewas 0.1 V, and a diode having low internal resistance was obtained.

In Example 16, a diode was formed using a pure Ga₂O₃ film. As a result,the forward voltage increased to 25 V. However, good results wereobtained for the dielectric breakdown field and the On-current.

Comparative Example 1

A Schottky barrier diode was produced in the same manner as in Example8, except that sputtering was performed using SiC as the target insteadof the Ga₂O₃-based material. The Schottky barrier diode showedrectifying characteristics to some extent, but a performance comparableto that achieved using Ga₂O₃:SnO₂ (=99.9:0.1 wt %) was not obtained. SiCis a promising material for a next-generation power device. However, itwas found that it is difficult to use SiC as a material for producing adiode when SiC is not epitaxially grown on a monocrystalline substrate.

Comparative Example 2

In Comparative Example 2, Mo having a small work function was used toform the Schottky electrode. As a result, diode characteristics were notobserved.

TABLE 2 Example 7 Example 8 Example 9 Example 10 Example 11 Schottkyelectrode Pd Pd Pd Pt Ni Work function (eV) of 5.5 5.5 5.5 5.6 5.6Schottky electrode Si wafer n-type n-type n-type n-type — Ohmicelectrode W Ti Ti Ti Ti Semiconductor Ga₂O₃:SnO₂ = Ga₂O₃:SnO₂ =Ga₂O₃:SnO₂ = Ga₂O₃:SiO₂ = Ga₂O₃:TiO₂ = composition (wt %) 99.9:0.1 wt %99.9:0.1 wt % 99.9:0.1 wt % 99.9:0.1 wt % 99.9:0.1 wt % SemiconductorGa:Sn = Ga:Sn = Ga:Sn = Ga:Si = Ga:Ti = composition (at %) 99.88:0.12 at% 99.88:0.12 at % 99.88:0.12 at % 99.84:0.16 at % 99.88:0.12 at %Annealing after film Air, 300° C., 1 h Air, 300° C., 1 h Air, 300° C., 1h Air, 400° C., 1 h Air, 200° C., 1 h formation of semiconductorAnnealing after SBD was Not Not Air, 400° C., 1 h Air, 350° C., 1 h Notformed performed performed performed Band gap (eV) 4.8 4.8 4.8 4.8 4.8Carrier concentration (cm⁻³) 5 × 10¹⁵ 5 × 10¹⁵ 5 × 10¹⁵ 5 × 10¹⁵ 7 ×10¹⁵ n-value 2.3 2.5 2.1 1.9 2.3 Dielectric breakdown field 1.1 1.5 10.8 0.9 Vb (MV/cm) Semiconductor XRD Amorphous Amorphous Micro- Micro-Amorphous measurement result crystalline crystalline Substrate Si waferSi wafer Si wafer Poly- Alkali- crystalline free glass Si wafer Contactelectrode Ti Ti Ti Ti Mo Work function (eV) of ohmic 4.5 4.1 4.1 4.1 4electrode Upper electrode None Au Au Au Au Forward voltage (V) 12 2.5 22.2 3.1 On-current (A/cm²) — >10 >10 >10 >10 Thickness (nm) 200 200 200200 200 Example 12 Example 13 Example 14 Example 15 Example 16 Schottkyelectrode Ni Ni Ni Pd Pd Work function (eV) of 5.1 5.1 5.1 5.5 5.5Schottky electrode Si wafer — — — n-type n-type Ohmic electrode Al—Nd TiTi Ti Ti Semiconductor Ga₂O₃:GeO₂ = Ga₂O₃:SnO₂ = Ga₂O₃:SnO₂ = Ga₂O₃:SiO₂= Ga₂O₃ composition (wt %) 99.99:0.01 wt % 92:8 wt % 90:10 wt % 99.9:0.1wt % Semiconductor Ga:Ge = Ga:Sn = Ga:Sn = Ga:Sn = Ga = 100 at %composition (at %) 99.91:0.09 at % 90.24:9.76 at % 88.8:11.2 at %99.88:0.12 at % Annealing after film Air, 300° C., 1 h Air, 300° C., 1 hAir, 300° C., 1 h Air, 300° C., 1 h Air, 300° C., 1 h formation ofsemiconductor Annealing after SBD was Not Not Not Air, 600° C., 1 h Notformed performed performed performed performed Band gap (eV) 4.8 4.6 4.44.8 4.8 Carrier concentration (cm⁻³) 5 × 10¹⁵ 1 × 10¹⁶ 1 × 10¹⁶ 5 × 10¹⁵3 × 10¹⁴ n-value 2.4 2.1 2.5 2.5 3.5 Dielectric breakdown field 0.9 0.70.2 0.05 1 Vb (MV/cm) Semiconductor XRD Amorphous Amorphous AmorphousPoly- Amorphous measurement result crystalline Substrate Polyimide PCprovided PC provided Poly- Si wafer with SiO₂ with SiO₂ crystalline Siwafer Contact electrode Mo Ti Ti Ti Ti Work function (eV) of ohmic 4 4.54.5 4.1 4.1 electrode Upper electrode Au Au Au Au Au Forward voltage (V)3 2.5 2.5 0.1 25 On-current (A/cm²) >10 >10 >10 >10 >10 Thickness (nm)200 200 200 200 200 Comparative Example 1 Comparative Example 2 Schottkyelectrode Pd Mo Work function (eV) of 5.5 4.4 Schottky electrode Siwafer n-type n-type Ohmic electrode Ti Ti Semiconductor SiC Ga₂O₃:SnO₂ =composition (wt %) 99.9:0.1 wt % Semiconductor Si:C = Ga:Sn =composition (at %) 50:50 at % 99.88:0.12 at % Annealing after film Air,300° C., 1 h Air, 300° C., 1 h formation of semiconductor Annealingafter SBD was Not performed Not performed formed Band gap (eV) 2.7 4.8Carrier concentration (cm⁻³) 5 × 10¹⁵ 5 × 10¹⁵ n-value 4 — Dielectricbreakdown field 0.2 Leakage occurred Vb (MV/cm) Semiconductor XRD Poly-Amorphous measurement result crystalline (c-SiC) Substrate Si wafer Siwafer Contact electrode Ti Ti Work function (eV) of ohmic 4.1 4.1electrode Upper electrode Au Au Forward voltage (V) 10.3 Leakageoccurred On-current (A/cm²) 0.1 Leakage occurred Thickness 200 200

Examples 17 to 23

A structure similar to that of Example 8 was evaluated while changingthe semiconductor material, the thickness of the semiconductor film, andthe type of substrate. The results are shown in Table 3. Thesemiconductor film was formed by sputtering. The thickness of thesemiconductor film formed in Example 17 was 200 nm, the thickness of thesemiconductor film formed in Examples 18, 20, and 22 was 1 μm, and thethickness of the semiconductor film formed in Examples 19, 21, and 23was 10 μm.

Note that “4H—SiC” in Table 3 refers to a hexagonal SiC substrate havinga 4-layer repeating structure, and “YSZ” in Table 3 refers to ayttrium-stabilized zirconia substrate.

TABLE 3 Example 17 Example 18 Example 19 Example 20 Example 21 Schottkyelectrode Ni Ni Ni Ni Ni Work function (eV) of 5.1 5.1 5.1 5.1 5.1Schottky electrode Si wafer n-type — — n-type — Ohmic electrode Ti Ti TiTi Ti Semiconductor Ga₂O₃ Ga₂O₃ Ga₂O₃ Ga₂O₃:TiO₂ = Ga₂O₃:TiO₂ =composition (wt %) 98:2 wt % 96:4 wt % Semiconductor Ga = 100 at % Ga =100 at % Ga = 100 at % Ga:Sn = Ga:Ti = composition (at %) 95.4:4.6 at %91.1:8.9 at % Annealing after film Air, 150° C., 1 h Air, 150° C., 1 hAir, 150° C., 1 h Air, 150° C., 1 h Air, 150° C., 1 h formation ofsemiconductor Annealing after SBD was Not Not Not Not Not formedperformed performed performed performed performed Band gap 4.8 4.8 4.84.7 4.7 Carrier concentration (cm⁻³) 3 × 10¹⁴ 3 × 10¹⁴ 3 × 10¹⁴ 4 × 10¹⁴6 × 10¹⁴ n-value 2.8 2.9 2.7 2.8 2.8 Dielectric breakdown field 2.5 2.42.4 1.8 1.6 Vb (MV/cm) Semiconductor XRD Amorphous Amorphous AmorphousAmorphous Micro- measurement result crystalline Substrate Si waferSapphire 4H—SiC Si wafer YSZ Contact electrode Ti Ti Ti Ti Ti Workfunction (eV) of ohmic 4.1 4.1 4.1 4.1 4.1 electrode Upper electrode AuAu Au Au Au Forward voltage (V) 2.8 2.4 2.2 2 2 On-current(A/cm²) >10 >10 >10 >10 >10 Thickness (μm) 0.2 1.0 10 1.0 10 Example 22Example 23 Example 24 Schottky electrode Ni Ni Ni Work function (eV) of5.1 5.1 5.1 Schottky electrode Si wafer n-type — — Ohmic electrode Ti TiTi Semiconductor Ga₂O₃:SnO₂ = Ga₂O₃:SnO₂ = Ga₂O₃:Al₂O₃ = composition (wt%) 95:5 wt % 90:10 wt % 90:10 wt % Semiconductor Ga:Si = Ga:Ti = Ga:Al =composition (at %) 96.8:3.2 at % 93.5:6.5 at % 83.0:17.0 at % Annealingafter film Air, 150° C., 1 h Air, 150° C., 1 h Air, 150° C., 1 hformation of semiconductor Annealing after SBD was Not Not Not formedperformed performed performed Band gap 4.6 4.4 5.2 Carrier concentration(cm⁻³) 2 × 10¹⁵ 4 × 10¹⁵ 8 × 10¹⁴ n-value 2.9 2.9 3.5 Dielectricbreakdown field 1.6 1.5 3.1 Vb (MV/cm) Semiconductor XRD AmorphousAmorphous Amorphous measurement result Substrate Poly- Alkali- Alkali-crystalline free glass free glass Si wafer Contact electrode Ti Mo MoWork function (eV) of ohmic 4.1 4 4 electrode Upper electrode Au Au AuForward voltage (V) 2.2 3.1 3.1 On-current (A/cm²) >10 >10 >10 Thickness(μm) 1.0 10 10

As described in detail above, it is possible to obtain rectifyingcharacteristics better than those of a Schottky barrier diode thatutilizes crystalline silicon by utilizing a gallium oxide-based materialand a device (e.g., sputtering device) that achieves high massproductivity. The diode according to the invention exhibits sufficientrectifying characteristics even when produced at a low temperature equalto or less than 300° C. Therefore, the diode according to the inventioncan be produced using a glass substrate, a resin substrate, and thelike.

INDUSTRIAL APPLICABILITY

The Schottky barrier diode element according to the invention maysuitably be used for an electric circuit, an electric apparatus, anelectronic apparatus, a vehicle, an electric vehicle, and the like forwhich high-speed operation and high-speed switching characteristics arerequired.

Although only some exemplary embodiments and/or examples of theinvention have been described in detail above, those skilled in the artwill readily appreciate that many modifications are possible in theexemplary embodiments and/or examples without materially departing fromthe novel teachings and advantages of the invention. Accordingly, allsuch modifications are intended to be included within the scope of theinvention.

1. A Schottky barrier diode element comprising an n-type or p-typesilicon (Si) substrate, an oxide semiconductor layer, and a Schottkyelectrode layer, the oxide semiconductor layer comprising either or bothof a polycrystalline oxide that comprises gallium (Ga) as a maincomponent and an amorphous oxide that comprises gallium (Ga) as a maincomponent.
 2. A Schottky barrier diode element comprising an n-type orp-type silicon (Si) substrate, an oxide semiconductor layer, and aSchottky electrode layer, the oxide semiconductor layer comprising apolycrystalline oxide that comprises gallium (Ga) as a main component.3. The Schottky barrier diode element according to claim 1, wherein theoxide semiconductor layer comprises gallium at an atomic percentage([Ga]/([Ga]+[total metal elements other than Ga])×100) of 90 to 100 at%.
 4. The Schottky barrier diode element according to claim 1, whereinthe oxide semiconductor layer is formed on the silicon substrate, andthe Schottky electrode layer is formed on the oxide semiconductor layer.5. The Schottky barrier diode element according to claim 1, wherein theSchottky electrode layer is formed on the silicon substrate, and theoxide semiconductor layer is formed on the Schottky electrode layer. 6.The Schottky barrier diode element according to claim 1, wherein theoxide semiconductor layer comprises at least one element selected fromSi, Ge, Sn, Ti, Zr, and Hf in a ratio of 0.01 at % to 10 at % based ontotal metal elements included in the oxide semiconductor layer.
 7. TheSchottky barrier diode element according to claim 1, wherein the oxidesemiconductor layer has a carrier concentration of 1×10¹⁴ cm⁻³ or moreand 1×10¹⁷ cm⁻³ or less at room temperature.
 8. The Schottky barrierdiode element according to claim 1, wherein the Schottky electrode layeris a metal thin film having a work function of 4.7 eV or more.
 9. TheSchottky barrier diode element according to claim 1, wherein the oxidesemiconductor layer is covered with an insulating film so that an edgeof the oxide semiconductor layer is not exposed.
 10. An electric circuitcomprising the Schottky barrier diode element according to claim
 1. 11.An electric apparatus comprising the Schottky barrier diode elementaccording to claim
 1. 12. An electronic apparatus comprising theSchottky barrier diode element according to claim
 1. 13. A vehiclecomprising the Schottky barrier diode element according to claim
 1. 14.A structure comprising a metal thin film having a work function of 4.7eV or more, and an oxide semiconductor that comprises Ga as a maincomponent, the structure having a region in which the metal thin filmand the oxide semiconductor electrically contact with each other. 15.The structure according to claim 14, wherein the oxide semiconductorthat comprises Ga as a main component comprises at least one elementselected from Si, Ge, Sn, and Ti in a ratio of 0.01 at % or more and 10at % or less based on total metal elements included in the oxidesemiconductor.
 16. The structure according to claim 14, wherein theoxide semiconductor comprises gallium at an atomic percentage([Ga]/([Ga]+[total metal elements other than Ga])×100) of 90 to 100 at%.
 17. The structure according to claim 14, wherein the oxidesemiconductor has a carrier concentration of 1×10¹⁴ cm⁻³ or more and1×10¹⁷ cm⁻³ or less at room temperature.
 18. The structure according toclaim 14, wherein the oxide semiconductor has a thickness of 50 nm to 20μm.
 19. The structure according to claim 14, wherein the metal thin filmis formed of Au, Cr, Cu, Fe, Ir, Mo, Nb, Ni, Pd, Pt, Re, Ru, W, In₂O₃,In—Sn—O, or In—Zn—O.
 20. An oxide semiconductor substrate comprising aconductive substrate, and the structure according to claim 14 that isstacked on the conductive substrate.
 21. The oxide semiconductorsubstrate according to claim 20, wherein the conductive substrate isformed of one or more materials selected from monocrystalline silicon,polycrystalline silicon, and microcrystalline silicon.
 22. An oxidesemiconductor substrate comprising an insulating substrate, and thestructure according to claim 14 that is stacked on the insulatingsubstrate.
 23. A power semiconductor element wherein the oxidesemiconductor substrate according to claim 20 is used.
 24. A diodeelement wherein the oxide semiconductor substrate according to claim 20is used.
 25. A Schottky barrier diode element wherein the oxidesemiconductor substrate according to claim 20 is used.
 26. A Schottkybarrier diode element comprising the oxide semiconductor substrateaccording to claim 20, the oxide semiconductor that comprises Ga as amain component being an oxide semiconductor layer, and the metal thinfilm having a work function of 4.7 eV or more being a Schottky electrodelayer.
 27. An electric circuit comprising one or more elements selectedfrom a group consisting of: (a) a power semiconductor element wherein anoxide semiconductor substrate is used, said oxide semiconductorsubstrate comprising a conductive substrate, and a structure that isstacked on the conductive substrate, said structure comprising a metalthin film having a work function of 4.7 eV or more, and an oxidesemiconductor that comprises Ga as a main component, said structurehaving a region in which the metal thin film and the oxide semiconductorelectrically contact with each other; (b) a diode element wherein anoxide semiconductor substrate is used, said oxide semiconductorsubstrate comprising an insulating substrate, and a structure that isstacked on the insulating substrate, said structure comprising a metalthin film having a work function of 4.7 eV or more, and an oxidesemiconductor that comprises Ga as a main component, said structurehaving a region in which the metal thin film and the oxide semiconductorelectrically contact with each other; and (c) the Schottky barrier diodeelement according to claim
 25. 28. An electric apparatus comprising theelectric circuit according to claim
 27. 29. An electronic apparatuscomprising the electric circuit according to claim
 27. 30. A vehiclecomprising the electric circuit according to claim 27.